I have been using sabnzbd for a couple years now. I upgraded to 5.2 when this issue started. My setup is as follows:
Windows 7 64 bit Professional
Sabnzbd 5.3
Sickbeard Alpha 522
I initially thought that it was Sickbeard causing the problem (it has a custom script). I disabled that and ran into the same problem.
When adding a nzb file, sabnzbd adds it to the queue and then downloads the files in a normal fashion. It appears to complete the PAR2 check and then the sabnzbd process goes up to 100% cpu usage (for some reason the sabnzbd.exe process is a high priority process which i didnt set in the config) right after unrar.exe starts. When I terminate unrar.exe through the task manager, it gives me this error message "Unpacking failed, [Error 299] Only part of a ReadProcessMemory or WriteProcessMemory request was completed". This seems reasonable since the unrar process was terminated. Logs don't seem much help but I'm happy to post whatever.
I tried reinstalling over the 5.3 installation again but I have the same problem. Any ideas

Version: 5.3 Final
OS: Win 7 x64
Install-type: Windows Installer
Skin (if applicable): Plush
Firewall Software: None
Are you using IPV6? No
Is the issue reproducible? Yes